Computer systems and television systems often simultaneously display images from two video sources. For example, a computer system may display a motion picture in one window, while presenting a control panel to control the display of the motion picture in another window. Televisions display images from two channels at the same time, as a picture-in-picture.
Standards, such as ITU-R-656, have been developed to allow for display devices to be interconnected. A device that renders a single image will have one input port, a device that can render two images will have two input ports, and so on. These same standards are used to interconnect other devices, such as video storage devices, video encoders, and the like, as shown in FIG. 1. For ease of discussion, the term rendering device is used herein to refer to any device that accepts video data that is suitable for rendering on a display. Shown in FIG. 1 is a video processor 100 that receives inputs from two video sources, Vid A 102 and Vid B 104, and produces, therefrom, two decoded outputs 112, 114. The video sources 102, 104 may be, for example, an MPEG stream and a conventional television signal, such as NTSC, PAL, or SECAM. The decoders 120, 140 convert the video sources 102, 104 into a digital form that is suitable for rendering systems such as displays 150 and 156, as well as a storage unit 152, or other processing devices such as an MPEG, PAL, or NTSC encoder 154. The display 150 displays images from multiple sources, as shown by the connection of decoded outputs 112, 114 to the multiple inputs of the display 150. Each decoded output 112, 114 has an associated clock signal 113, 115, respectively, for controlling the display of the images associated with each of the decoded output signals. The decoding of signals from varying sources can occur at differing frequencies. For example, MPEG decoding conventionally produces decoded output at a frequency of 27 MHz; television decoding produces outputs at approximately 28 MHz, 36 MHz, and other frequencies, depending on the format used to encode the video streams. These frequencies produce the appropriate number of data values for a display of each image frame at the appropriate frame rate.
The advantage of a standard interconnection scheme is that any device that conforms to the standard can be interconnected with any other device that also conforms to the standard. In FIG. 1, for example, by using a standard based upon a single-port configuration, devices 152, 154 that receive a single video input and a device 150 that receives multiple video inputs are compatibly interconnected to a video processor 100 that provides multiple video outputs.
As can be seen in FIG. 1, the interconnection among the devices in a single-port configuration requires a separate connection for each decoded output 112, 114. Typically, the connections contain multiple parallel data lines. For example, ITU-R-656 calls for 8 data lines for each port. The video processor 100 in this example requires 16 data lines, as does the video display device 150. If more than two video sources are decoded, 8 data lines are required for each port. This multiplicative effect is particularly problematic when multiple decoders are integrated into a single integrated circuit, because the number of pins required for an integrated circuit has a direct impact on manufacturing costs and reliability.
Another problem with the use of individual ports for each decoded output is the complexity of interconnection. The single port devices 152 and 154 are directly connected to one of either output ports 112, or 114, exclusively. A rewiring of the interconnections to the video processor is required to allow, for example, the video encoding device 154 to receive the decoded signal 112 corresponding to Vid A 102. Alternatively, the rendering device 156 is interconnected via a switch 170 to either output ports 112, 114, to allow for ease of switching. Note, however, that switch 170 requires the switching of 8 data lines and a clock line to each of the available output ports 112, 114 and their associated clocks 113, 115. Again, this multiplicative effect is particularly problematic for integrated circuit based designs, as well as for systems designed for multiple video source processing, such as studio systems.
Therefore, a need exists for a method and apparatus for combining video streams that minimizes the number of interconnections required between devices, while also providing for compatibility among devices of varying capabilities.